1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material, and more particularly to a reversible thermosensitive recording material which utilizes a coloring reaction of an electron donating coloring agent and an electron accepting coloring developer and in which a colored image is repeatedly formed and erased by appropriately heating and cooling the reversible thermosensitive recording material.
2. Discussion of the Related Art
A variety of thermosensitive recording materials are well known in which a colored image can be formed by a coloring reaction when an electron donating coloring agent is brought into contact with an electron accepting coloring developer while heat or the like is applied thereto.
The thermosensitive recording materials have the following advantages over other conventional recording materials:
(1) color images can be rapidly recorded by a relatively simple apparatus without using such complicated steps as developing and fixing;
(2) color images can be recorded without producing noise and environmental pollution;
(3) various color images, e.g., red, blue, violet and black, can be easily obtained;
(4) image density and background whiteness are high; and
(5) the manufacturing cost is relatively low.
Because of these advantages, the above-described thermosensitive recording materials can be widely used, not only as a recording material for price labels in stores, but also as recording materials for copiers, printers for computers, facsimiles, automatic vending machines of tickets, video printers and measuring instruments.
However, the coloring reaction of these thermosensitive recording materials is irreversible and accordingly the used recording materials cannot be repeatedly used by erasing the recorded images.
Several thermosensitive recording materials which can reversibly form and erase an image have been proposed. For example, they are as follows:
(1) a thermosensitive recording material which includes gallic acid in combination with phloroglucinol serving as coloring developers (Japanese Laid-Open Patent Publication No. 60-193691);
(2) a thermosensitive recording material which includes phenolphthalein or Thymolphthalein serving as a coloring developer (Japanese Laid-Open Patent Publication No. 61-237684);
(3) a thermosensitive recording material which includes an uniform solid solution of a coloring agent, a coloring developer and a carboxylic acid ester (Japanese Laid-Open Patent Publications No. 62-138556, 62-138568 and 62-140881);
(4) a thermosensitive recording material which includes an ascorbic acid derivative serving as a coloring developer (Japanese Laid-Open Patent Publication No. 63-173684); and
(5) a thermosensitive recording material which includes bis(hydroxyphenyl)acetic acid or a higher aliphatic amine salt of gallic acid serving as a coloring developer (Japanese Laid-Open Patent Publication No. 2-188294).
However, these reversible thermosensitive recording materials have one or more of the following drawbacks:
(1) at least one of (a) the stability of image formation and (b) the stability of image erasure is not satisfactory;
(2) the image density of the recorded image is not satisfactory; and
(3) image qualities deteriorate after images are repeatedly formed and erased.
Therefore, these reversible thermosensitive recording materials are not in practical use.
In attempting to solve these problems, the present inventors have proposed a reversible thermosensitive recording material which includes a coloring agent such as a leuco dye and a coloring developer such as an organic phosphate compound having a long-chain aliphatic hydrocarbon group, an aliphatic carboxylic acid compound or a phenolic compound (Japanese Laid-Open Patent Publication No. 5-124360). The reversible thermosensitive recording material can stably repeat image formation and image erasure by being appropriately heated and cooled, and the image-recorded state or the image-erased state can be stably maintained in room temperature. Japanese Laid-Open Patent Publication No. 6-210954 has disclosed a reversible thermosensitive recording material which includes a specified phenolic compound (having a long-chain aliphatic hydrocarbon group) serving as a coloring developer, and is as good as the above-mentioned reversible thermosensitive recording material.
These reversible thermosensitive recording materials, however, have a drawback in that the image density deteriorates or the recording layer becomes deformed when images are repeatedly formed and erased in various environmental conditions of practical use of the recording materials. The reason for the problem is considered to be that the structures of the recording layer and the protective layer of the recording material are gradually changed and deteriorated by the mechanical force and the heat of a thermal printhead which applies heat to the recording material to form or erase images.
In attempting to solve this problem, a reversible thermosensitive recording material has been disclosed which has a recording layer including particles whose average particle diameter is more than 1.1 times the thickness of the recording layer (Japanese Laid-Open Patent Publication No. 6-340171). In addition, a reversible thermosensitive recording material has been disclosed which has good ability to be used with thermal printheads owing to the formation of a protective layer thereon which has specified gloss and surface smoothness (Japanese Laid-Open Patent Publication No. 8-156410). In addition, in attempting to provide a reversible thermosensitive recording material which can repeatedly and stably record and erase images, Japanese Laid-Open Patent Publications No. 5-124360 and 6-344659 respectively disclose a recording material including a crosslinked resin which is formed upon application of heat, ultraviolet light or electron beams, and a method to irradiate a recording layer with electron beams. However, these reversible thermosensitive recording materials cannot yet entirely improve the deterioration of the recording layer and the protective layer, resulting in occurrence of deformation of the recorded image when the recording materials are repeatedly used.
Because of these reasons, a need exists for a reversible thermosensitive recording material which has good abilities to repeatedly form and erase images and can maintain good image qualities such as good image contrast and good image sharpness without deformation of the recording material-even when the recording material repeatedly forms and erases images.